Stable isotope dilution gas chromatographic-mass spectrometric (GC-MS) measurement of tricyclic antidepressants (TCA) is a useful alternative to high performance liquid chromatographic (HPLC) methods when interfering substances prevent accurate quantitation by HPLC. For satisfactory GC-MS analysis, secondary amine TCA must be derivatized. Commonly employed trifluoroacetyl and heptafluorobutyryl derivatives are relatively unstable and cause rapid deterioration of capillary gas chromatography (GC) columns. We have therefore examined 4-carbethoxyhexafluorobutyryl chloride (CHFB-Cl) as an alternate derivatizing agent and have developed a stable isotope dilution GC-MS method employing ring-labeled [2H4]-desipramine and [2H4]-imipramine internal standards which permits measurement of desipramine, nortriptyline, imipramine, and amitriptyline in plasma samples containing one or all of these analytes. The GC-MS assay is linear for each analyte from the lower limit of quantit ation (25 ng/mL) up to 1500 ng/mL and correlates well with HPLC measurements. The GC-MS analytic coefficient of variation was 9.7 1 1.3% for all analytes considered together. Although interferences are observed in the HPLC assay, thioridazine, perphenazine, cyclobenzaprine, or norcyclobenzaprine do not interfere with GC-MS measurements of the TCA examined here. The stability of the CHFB derivative of secondary amine TCA was found to be superior to that of the trifluoroacetyl derivatives of these compounds.